618 THE ANIMAL KINGDOM. 



may be disposed in parallel bands, sometimes connected by anastomotic 

 bundles, e. g. in Sipunadus, some species of Phascolosoma ; or one layer 

 only may be so disposed, e. g. the circular muscles of Priapulus and Hali- 

 cryptus ; or all the layers may be continuous, e. g. in Echiurus. But 

 species in the same genus may vary in this respect. The musculature is 

 covered internally like all the organs of the body by a peritoneal epithelium 

 which may be uniformly ciliated (Sipunculus, Phascolosoma]. 



The nervous system consists of a median ventral cord, and of a 

 pharyngeal ring, which extends in the Echiuridae into the proboscis, 

 coursing along its edges. In Priapulus and Halicryptus the ring and cord 

 are in continuity with the hypodermis, in which the nerves also run ; in 

 other cases they lose this connection, and lie internally to the muscular 

 layers. The cord is never ganglionated, and it is only in the Sipunculidae 

 that a bilobed supra-oesophageal swelling is developed. In Echiurus and 

 Thalassema it is traversed by a fine canal. The ganglion cells are evenly 

 distributed either as two lateral bands, or more rarely as a single ventral 

 band. The nerves originate as a rule from the cord opposite, or nearly 

 opposite to one another, and in Sipunculus nudus, Echiurus Pallasii, Tha- 

 lassema erythrogrammon, they unite dorsally, thus forming complete rings. 

 The cord may bifurcate posteriorly, as in Sipunculus nudus 1 . A plexus of 

 nerve-fibres has been observed in the same animal in the cutis, with 

 terminal branches in connection with small groups of elongated hypodermis 

 cells. A fine pore may perforate the cuticle above these groups. The 

 integumental spines of the Priapulidae are cuticular elevations lined with 

 elongated hypodermis cells, and in Priapulus caudatus, according to Scharff, 

 some or all of the cells end in projecting sensory hairs 2 . The papillae of 

 Echiuridae consist of thickenings of the cutis, into which a nerve pene- 

 trates and branches ; its branches have interpolated ganglion cells, and end 

 in the ordinary hypodermis cells (Greeff). Two or more pigment specks, 

 representing eyes, lie on the supra-oesophageal ganglion in Phascolosoma. 

 Eye-specks are also present in Phymosoma. 



The mouth is anterior and terminal in the Sipunculidae and Pria- 

 pulidae, at the base of the prostomium and surrounded by it in Echiuridae. 

 The anus is dorsal, and more or less anterior in the Sipunculidae, terminal 



1 There is a space between the two sheaths of the ventral nerve-cord in Sipunculus nudus, 

 filled in prepared specimens by a coagulum with nuclei and pigment clumps in suspension, but no 

 vascular elements. Hence it is not a blood-vessel (Andreae). A vessel or blood-space is said by 

 Greeff to inclose the nerve-cord of Echiurus, but Spengel, who investigated the same animal, did not 

 find anything of the kind, and professes himself unable to explain Greeffs figures. In addition to 

 the facts given in the text, Spengel states that in Echiurus transverse nerves pass across the proboscis 

 from one side of the nerve-ring to the other, and that in the young animal the cord has a ganglionated 

 aspect, but the interganglionic gaps are irregular. See p. 624 post. 



2 The wart-like papillae which cover the dorsal aspect at the posterior extremity of the body in 

 Priapulus caudatus are glandular, according to both Apel and Scharff. 



